1. Field of the Invention
The present invention relates to a connecting member of a construction machine such as an excavator.
2. Description of the Related Art
An excavator, which is a construction machine, comprises a travel device, a main body (a slewing structure) and an attachment (working device). The attachment comprises a boom, an arm attached to the leading end of the boom, and a bucket attached to the leading end of the arm. The leading end of the boom is provided with a boom top (a connecting member), for interconnecting the boom and the arm. The boom top comprises two brackets and an intermediate support member between the brackets.
Although conventional boom tops have employed mainly welded structures (structures in which several parts are interbonded by welding), recently boom tops are often formed by casting. Forming boom tops by casting requires no welding operation, thereby enabling an efficiency in productivity and operational to be improved and allowing the boom top to be shaped to a complex form (this results in allowing the thickness of the various sections of the boom top to be designed in consideration with both local stress concentration and weight reduction). Japanese Patent Application Laid-open No. 2004-108055 discloses a conventional arm-mounting member (boom top) formed by casting.
FIG. 15 is a perspective-view cross-sectional diagram showing an example of a conventional boom top 901. The cross section shown in FIG. 15 corresponds to one in V-V shown in FIG. 3. The boom top 901 has two brackets (FIG. 15 shows only one bracket 911) and an intermediate support member comprising a cover plate 922 and a bottom plate 921. The bracket 911 is formed with a shaft hole 911h, into which an arm pin is inserted.
In conventional boom tops, the member between the two brackets (the member is the cover plate 922 in FIG. 15) prevents liquid or the like from entering into the boom.
Furthermore, for weight reduction, the intermediate support member of the boom top (the member comprises a cover plate 922 and a bottom plate 921 in FIG. 15) is formed hollow (refer to FIG. 15 showing an inner space 902s). Hence, forming the boom top by casting requires an inner casting mold in addition to an outer casting mold. The boom top further needs a hole for drawing out the inner casting mold (i.e., a draw-out hole). In general, the draw-out hole is formed in the bottom plate (in FIG. 15, a draw-out hole 921h is formed in the bottom plate 921).
During the operation of the excavator, an unbalanced load acting on the bucket generates a torsional load acting on the boom and the boom top. Besides, the draw-out hole formed in the bottom plate to remove the casting mold lowers the strength of the periphery of the hole (the “periphery” is the portion Z enclosed by a broken-line circle shown in FIG. 15). When the torsional road acts on the boom top to generate a stress concentration in the periphery, the bottom plate may be brought into fracture.
For this reason, a conventional boom top is provided with a plate as a capping part, which is welded to the edge so as to plug the draw-out hole, in order to reinforce the bottom plate (the “capping part” is shown as a cap 970 in FIG. 15).
However, as a first problem, this case involves a welding operation for the capping part, which increases the time and costs for manufacturing the boom top.
Moreover, as a second problem, the attached capping part increases the weight of the boom top, thus causing a requirement of a heavier counterweight. For this reason, a lighter boom top is desired.